1. Field of the Invention
The invention relates to an automatic transmission for a vehicle and, more particularly, to an automatic transmission having a clutch operated by a hydraulic servo with a stationary type cylinder for shifting gear stages.
2. Description of the Prior Art
Generally in automatic transmissions for vehicles, at least two clutches are required for engaging and disengaging rotary members to shift gear stages, irrespective of the structure of the gear train in the transmission. In the prior art, the automatic transmission clutches are a wet type multiple disk clutch, in which both the clutch hub and the clutch drum supporting the friction plate portions are fixed to rotary members. It is customary that the hydraulic servo for bringing the friction plate portion into engagement be integrated with the clutch drum. This structure (called the "clutch drum type") has a cylinder formed in the rotating clutch drum, with a piston slidable in the cylinder to define an oil chamber. The clutch of this structure is exemplified by Japanese Patent Laid Open No. 52249/1987.
In contrast to the prior art clutches in automatic transmissions, wet type multiple disk brakes in prior art automatic transmissions have stationary reaction elements mounted on the transmission case or the like.
It is desirable to reduce the size of automatic transmissions and particularly the rotating members. However, the clutch drum type structure, as described above, limits the ability to reduce the size of the transmission because the rotary members must accommodate the cylinders of the hydraulic servos. This, and the requirement for at least two clutches for establishing a plurality of forward gear stages, prevent size reduction of the automatic transmission.
The rotary members in an automatic transmission undergo a change in rotational speed during a shift operation. Forces from inertia (inertia torque) produced by the rotational speed change and the weight of the rotary members result in fluctuations of the output shaft torque to cause shift shocks. In automatic transmissions employing clutches of the clutch drum type in which the hydraulic servos are contained in the rotary members, the weight of the rotary members is increased by the hydraulic servos to increase the inertia torque, thus increasing the severity of shift shocks.
Additionally, the prior art hydraulic servo of the clutch drum type produces a centrifugal oil pressure by centrifugal force acting upon the oil in the servo cylinder. This centrifugal oil pressure adds to the oil pressure from the hydraulic control system on the piston so that the oil pressure characteristics are changed during the shift operation by the raised oil pressure. Specifically, the centrifugal oil pressure acts to accelerate the release of the clutches being released and to quicken the engagement of clutches being engaged during the shift operation. For these reasons, the prior art includes an oil chamber for offsetting the centrifugal oil pressure as exemplified in Japanese Patent Laid Open No. 52249/1987. However, this provision of the centrifugal oil pressure offsetting chamber tends to further increase shift shocks because of an increase in the weight of the rotary members. Further the provision of the centrifugal oil pressure offsetting chamber enlarges the size of the system.
In the case of the hydraulic servo of the clutch drum type of the prior art, the inside of the rotating cylinder is fed with an oil pressure from a stationary member such as a transmission case so that a seal member is required for preventing oil leakage from between the relatively rotating portions of the cylinder and the stationary member. However, the seal member establishes a dragging loss to lower the transmission efficiency of the automatic transmission.
In order to eliminate such sliding resistance and to thereby prevent generation of excessive centrifugal oil pressure, therefore, it is effective to provide a stationary servo cylinder, separate from the clutch drum, so that only the servo forces are applied to the engagement element. When this design is adopted, the cylinder is fixed to structure which does not rotate, such as the transmission case. If the cylinder is formed in the transmission case, the overall structure can be made more compact because it requires neither an additional stationary member nor a separate cylinder wall member.
In an automatic transmission, however, two or three sets of clutches are usually required for the speed change mechanism. Even in the case of a speed change mechanism requiring only two sets of clutches, these two clutches are not always arranged at the end portions of the speed change mechanism. Despite the aforementioned advantage, however, there has never been realized, in actual practice, an automatic transmission structure in which the hydraulic servos of the clutches are of the stationary cylinder type formed in the transmission case. Moreover, when the hydraulic servos of the clutches are of the stationary cylinder type, their oil pressures are not balanced in the clutch drum, thereby creating another problem in that a reaction force receiving structure is required for bearing the hydraulic servo forces.
However, clutch servos having their cylinders formed in the two end walls of the case have been used in the prior art in a torque distribution mechanism, i.e. the differential unit of a vehicle, which requires that the clutches be disposed in the final output unit of the mechanism and be arranged symmetrically with respect to the case. These clutches are exemplified by those disclosed in Japanese Patent Application Laid-Open ("Kokai") No. 248721/1990 and Japanese Patent Application Laid-Open ("Kokai") No. 241287/1994.
The reaction force receiving structure in the torque distributing mechanism of Kokai No. 248721/1990 is disposed at the inner circumferential side of the cylinder and uses a bearing so as to restore the hydraulic servo force at the same case end wall in which the servo cylinder is formed. In this arrangement, the cylinder and the bearing are radially overlapped to limit the diameter of the cylinder and to thereby allow a larger hydraulic servo diameter. On the other hand, Kokai No. 24128/1994 discloses reaction receiving structure in which a step portion is formed radially outward of the clutch drum and in which the bearing is arranged between the step portion and the inner circumference of the case, so that the hydraulic servo force may be transmitted to and borne by the case. In this latter design, although the bearing is radially enlarged, it can give satisfactory service when the rotation is at a relatively low circumferential speed, equal to that of the rotation of the axle, as in the case of the clutch of the differential unit. If, however, this latter design is applied to structure rotating at a relatively high speed, e.g. structure transmitting the engine rotation substantially as is, as in the clutch for connecting the input shaft of the automatic transmission and the speed change mechanism, the circumferential speed of the bearing is so high that large-sized bearings with matching load capacity must be used to provide the required durability. Thus, this latter design cannot be adopted consistent with the objective of a compact transmission. So long as the concept of the prior art for receiving the hydraulic servo force in the vicinity of the hydraulic servos is thus followed, it is impossible to realize effective support for bearing the force of the hydraulic servo in an automatic transmission.